This invention relates to a method of forming multilayer coatings having metallic glamor.
The exterior of automobile bodies, for example, is finished with a metallic base coating and a clear top coating formed on the base coating for decorative and protective purposes. For higher productivity, the clear top coating is conventionally applied on the base coating wet-on-wet and cured simultaneously with the base coating. This method is highly suitable for in-line coating operation in the automobile industry and gives a high grade finish in terms of appearance, weatherability, solvent and chemical resistances, discoloring resistance and the like.
In order to achieve excellent appearance, particularly excellent metallic glamor, it is imperative that the top coat applied on the base coat wet-on-wet does not cause intermixing of the two layers which, if it occurs, greatly impairs the orientation of metallic flakes and the metallic glamor. For this reason, attempts have been made to decrease the compatibility between the base coat and the top coat by, for example, using a resin having a higher molecular weight for the base coat than for that of the top coat or by using different resins for different coats such as the combination of acrylic top coat/polyester or cellulose acetate butyrate base coat. The compatibility between the uncured two coats may also be decreased by modifying the coating conditions thereof. This technique includes two-stage application of the base coat, prolonged rest intervals between application steps, elevation of the viscosity the of base coat relative to the top coat and the like. However, none of these known attempts is completely satisfactory. The use of high molecular weight resins requires a decrease in their solid contents at the time of application. When different resins are used for different coats, the adhesion between different coats is decreased. Modification of coating conditions increases the number of steps and the length of time required for the overall coating operation.
One approach for improving aesthetic properties of multicoat system is to provide a relatively thick top coat on the base coat. In a two coat system comprising a base coat containing aluminum flakes of 10 to 50 .mu.m size, large aluminum flakes often protrude above the base coat surface. The clear top coat therefore must have a film thickness sufficient to compensate for these protrusions. However, with conventional top coat compositions, the film thickness is limited to only 20-30 .mu.m with a single coating operation, or 40-45 .mu.m with two coating operations. This is because conventional coating compositions tend to excessively run with an increase of the amount applied per unit area. Thick top coats may be provided by multiple coating operations. However, this technique is less efficient and requires an extensive modification of existing production lines.
Recently, with the objective of economizing natural resources and energy and because of the requirements for pollution control, much research has been conducted with the objective of for increasing the nonvolatile contents of coating materials. High-solids coating systems are generally formulated by lowering the molecular weight of vehicle resins but this technique, when applied to two coat systems to be applied wet-on-wet, presents several serious problems, such as poor metallic flake orientation, intermixing, poor gloss, excessive run and the like. Another approach would be to incorporate a non-aqueous resin dispersion into the system. However, experiments have shown that this method suffers from the above-mentioned problems because the increase in viscosity after application takes too long time.
We have already proposed in Japanese Laid Open Patent Application No. 60-94175 published May 27, 1985 to incorporate internally crosslinked polymer microgel particles of 0.01 to 10 .mu.m size into both the base coat and top coat compositions. By incorporating the polymer microgel particles, the composition exhibits a yield point such that when a shear force above the yield point is exerted, the composition may be easily fluidized. Once deposited on a substrate, the composition exhibits a high structural viscosity. For this reason, migration of metal flakes in the base coat due to the convection of solvent, intermixing of the two coats and run are prevented, thereby ensuring an excellent finish having improved gloss and other aethetic properties even when the top coat is applied wet-on-wet in a relatively large film thickness.
When relatively low molecular weight resins are used as a vehicles resin in the above multilayer coating system for achieving high solid contents, an acid catalyst is required for accelerating the curing reaction thereof because such vehicle resins are generally less reactive with a cross-linker than higher molecular weight resins.
The use of such acid catalyst is often undesirable because it tends to impair the storage life of the coating compositions. When used excessively, the acid catalyst remains in the finished coating and adversely affects the quality of the finished coating.